You could, I suppose, do something like leave a note for your future self saying "you need to go back in time and do this", but that's rather pointless. In general, either you can change the past, in which case it's usually a branching-timeline setup and you don't actually need to go back in time after you've changed things, or you can't (Stable Time Loop mode) in which case you can't really prevent disasters with time travel. (Or at least you're doing so from the perspective of a world in which the disaster was successfully prevented already.)

Well, a lot of time travel stories make it so the universe sort of automatically prevents itself from having paradoxes. So you might go back in time and save someone from being killed by a hit-and-run car, only for them to be mugged and killed a week later to make sure the same-ish outcome happens. Or if you're trying to kill someone, everything goes wrong, you're gun misfires, someone shows up to save them at the last minute, they survive things that have a 1 in a 100 change of survival. Stuff like that.

So in the case of going back in time to kill your grandfather before he can have kids that would result in you, you just couldn't, everything would contrive to get in your way.

edited 3rd Dec '11 5:21:16 PM by NoirGrimoir

SPATULA, Supporters of Page Altering To Urgently Lead to Amelioration (supports not going through TRS for tweaks and minor improvements.)

And, probably, you would find out a while later about some old family stories about how your grandfather once almost got killed a bunch of times and survived by insane luck. (This is one thing that bugs me about single-timeline stories in which only the big, obvious events need to be the same. Everything has to be the same, because in reality it's all quantum, and on a quantum level your grandfather dying or not dying is no more 'important' than the precise reflection paths of the sunlight that hit the Earth on his birthday.)

What kind of timeline do you imagine? One timeline like Back to the Future? You can easily avoid the grandfather paradox if you imagine it as multiple universes splitting off from a single event. One universe - grandfather dies (but you live, because you caused a split, and your grandfather's still alive when you're from), but in another universe, Grandfather lives (and you're born).

This is one thing that bugs me about single-timeline stories in which only the big, obvious events need to be the same. Everything has to be the same, because in reality it's all quantum, and on a quantum level your grandfather dying or not dying is no more 'important' than the precise reflection paths of the sunlight that hit the Earth on his birthday.

Yes, I hate all time-travel except where going back in time creates another dimension, because it's complete bull and makes me think the writer doesn't know what they're talking about. To make you took a very specific sperm and egg, delaying or changing anything even remotely could totally mess that up.

edited 3rd Dec '11 6:20:59 PM by NoirGrimoir

SPATULA, Supporters of Page Altering To Urgently Lead to Amelioration (supports not going through TRS for tweaks and minor improvements.)

In the case of splitting timelines, or the essentially-equivalent destructive non-causally-looping single timeline, you can do whatever you want just as if you hadn't traveled at all. In the case of a single causally looping timeline, whence cometh stable time loops, you couldn't kill your grandfather because either the universe will conspire to avoid it or you're predetermined not to (and if you take quantum effects into account, per my last post, they end up being more or less equivalent).

When writing any time travel story at all, one must remember: Either the past can be changed, or it can't.

If the past can't be changed, that means that anything you do in the past will have already happened. You could go back and try to kill your grandfather, but the fact that you were born means that you either failed to or decided not to.

If you can change the past, then anything you do in the past will affect the "present" that you return to. If you go back in time and kill your grandfather, history will change accordingly. You won't disapear, though, because you're from another timeline. The act of changing the past created a new timeline, and the timeline you came from wasn't changed—it was erased, but you still exist because you weren't in it at the time.

The thing that always bugs me about the "you change something and make and alternate timeline" is that you've effectively created an entire new universe.

If the Earth of this timeline is not the same as the original Earth from the original timeline, then nor is the sun it orbits the same sun, nor is it the same galaxy - yet this alternate Earth has a sun, neighbouring planets, comets etc. It's in a galaxy which is one of millions of billions in this amazing and expanding universe [/song]

In the instant you make a change, an alternative universe comes into existence as a parallel "timeline". Every single thing in this universe is identical to the one just "left" - right down to quantum level and vectors, spins etc - right up to the POD. From the POD, most this other universe keeps on going pretty much as the original (your change can't even affect Alpha Centauri for at least another four years (if at all), let alone anywhere else). From the POD, the change sends its ripples outwards, making this alternative Earth progressively more different from the original.

It's that "creating an entire new universe" aspect that I can't buy.

I can't recall the estimated mass present in our universe but we'll just call it a fuckton.

Which means that the energy required just to create the mass alone = fuckton*c^2 which is even more.

How could you change an event, thus creating an entire alternate dimension with its own universe? There's a limit to how much energy you can get out of putting a bullet in someone's brainpan.

If you buy many-worlds quantum, which seems to be the only theory that makes any fucking sense, then it's already happening constantly anyway and time travel just needs to pick one of those branchings.

In the "proper" version of many-worlds, there's only one "world;" it just contains within it all possibilities of any given event, in certain proportions. (This turns out to be important because they interfere with each other)

Actually, if you upgrade the "time machine" concept slightly, that would let your machine deposit you at any place at any time in any reality you like, For Want of a Nail being in as much force as you like. This includes absolutely insane ones, up to and including full-blown Cloudcuckooland.

Well, it's all strange and gets down to the level where things aren't things anymore, but I was under the impression that you could more or less visualize the separate probability blobs corresponding to the different outcomes as different universes. Is this wrong?

Perhaps the situation below where time traveling causes a person to time travel into another universe instead of their native universe.

A person from Universe 1 goes to the past of Universe 2 to prevent a disaster that occurred in Universe 2 past. But convinces himself from Universe 2 to go to the past of Universe 1 to prevent a disaster that occurred in Universe 1 past and to tell his younger self to go back to the past of Universe 2 to continue the Stable Time Loop forever.

I don't want to be an ass, but if you aren't sure about it, then don't use it to guess a solution to a thread that is over 3 years old. Answers that are just guesswork are, with threads like this, pretty much useless, and reviving a thread dead for 3 years just to give an answer of that sort is kind of improper.

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